The present invention relates to an insulated gate field effect transistor (hereinafter abbreviated to IGFET), and more particularly to an IC using a complementary type IGFET's (hereinafter abbreviated to CMOS) or to an IC using a bipolar and a CMOS (hereinafter abbreviated to BiCMOS).
The CMOS and the BiCMOS have been widely used in IC's, because the former has a low power consumption characteristic and the latter has a good high frequency and a low power consumption characteristics. Both IC's employ an N-channel type IGFET formed in a P-type substrate region, that is, a P-well, and a P-channel type IGFET formed in an N-type substrate region, and these N-type and P-type IGFET's are separated from each other by interposing an isolation region therebetween. The isolation region is made of a thick silicon oxide layer formed by thermal oxidation of a silicon substrate under a high temperature during a long time. In this case, at portions of the channel region of the IGFET abutted against the thick silicon oxide layer, a designed threshold voltage cannot be obtained, because impurities in both end portions of the channel region in the channel width direction abutted against the thick field silicon oxide layer migrate into the thick field silicon oxide layer and the impurity concentration at the end portions fall unfavorably to a low level. Particularly, borons are more apt to migrate, and the degree of the change in the boron concentration at the end portions of the channel region is largely deviated by the process conditions. Therefore, the problem is more serious in an N-channel type IGFET formed in a P-well doped with borons as the impurity. Consequently, in a prior art, a precise control of the characteristics of the IGFET or the IC using the IGFET is impossible.